How Does Cooking Affect the Amount of Vitamin C in Lemon Juice?

Hypothesis:

The longer the lemon juice is cooked up to the boiling temperature, the lower the concentration of vitamin C.

Variables:

1. Independent: Type of lemon juice samples [fresh lemon juice (0 minute boiled) , 10 minutes boiled lemon juice, 1 hour boiled lemon juice].

2. Dependent: The concentration of vitamin C.

3. Constant:

1. The volume of DCPIP solution used. In each trial, 1 cm3 of DCPIP solution is used. 1 cm3 is chosen because it is neither too much nor too little since too much DCPIP solution used will need high amount of lemon juice to decolourise. In fact, the volume needs to be fixed as different volume of DCPIP solution requires different volume of lemon juice to decolourise.

2. The volume of lemon juice used for dilution. 4 cm3 of each type of lemon juice is used to make up 100 cm3 solution. The volume must be constant as if different amount of lemon juice is used in the dilution, then there is no point of conducting the experiment since the data for each condition of lemon juice does not correspond and tally to each other.

3. The volume of glacial acetic acid used. 4 cm3 of glacial acetic acid used for dilution in all types of lemon juice.

4. Type of fruit used to prepare juice sample. Juices are extracted from lemon fruits for every trial. This is because different fruits contain different concentration of vitamin C, thus using variety of fruits will greatly affect the reliability of the data.

Apparatus & Materials / Procedures

Refer to the handouts.

DATA COLLECTION AND PROCESSING

1. Data Collection

1. Qualitative Data

1. Glacial acetic acid is a colourless solution. It releases pungent and choking smell.

2. Lime juice is a yellowish solution and releases tangy smell.

3. DCPIP is a dark blue solution. When lime juice is titrated into DCPIP, the DCPIP gradually changes from dark blue to pink and eventually decolourises.

4. The resulting solution at the end of the titration, which comprises of DCPIP, lime juice and glacial acetic acid is a cloudy mixture.

1. Quantitative Data

Lemon Juice sample

Volume of DCPIP (± 0.1 cm3)

Trial

Reading of burette (± 0.05 cm3)

Initial

Final

Fresh lemon juice

1.0

1

1.30

34.10

1.0

2

1.10

33.50

10 minutes boiled lemon juice

1.0

1

3.20

45.10

1.0

2

4.10

44.70

1 hour boiled lemon juice

1.0

1

1.90

34.80

1.0

2

1.30

28.50

Table 1 The initial and final readings of burette in the titration.

1. Data Processing

The volume of lemon juice sample used in the titration can be calculated by using the formula:

Volume of lemon juice sample used = final reading – initial reading

Uncertainty of volume of lemon juice used = uncertainty of final reading + uncertainty of initial reading

Below is an example of calculation made:

The volume of lemon juice sample used in trial 1 for fresh lemon juice = (45.10 – 3.20) cm3

= 41.90 cm3

Uncertainty = (0.05 + 0.05) cm3

= ± 0.10 cm3

The data calculated can be presented in following table:

Lemon Juice Sample

Trial

Volume of Lemon Juice Sample Used (± 0.10 cm3)

Fresh Lemon Juice

1

32.80

2

32.40

10 minutes boiled lemon juice

1

41.90

2

40.60

1 hour boiled lemon juice

1

32.90

2

27.20

Table 2 the volume of lemon juice sample used in each trial.

Next, we shall need to calculate the average volume of lemon juice sample used. Below is an example of calculation made:

Average volume of fresh lemon juice sample titrated = trial 1 + trial 2 2

= 32.80 cm³ + 32.40 cm³2

= 32.60 cm3

Uncertainty = ∑Ç x-x* Ç2 where x = volume of lemon juice sample titrated

x* = average volume of lemon juice sample titrated

= Ç32.80 cm3- 32.60 cm3Ç + Ç(32.40 cm3- 32.60 cm3)Ç2

= ± 0.20 cm3

Therefore, the average value of all lemon juice samples with their respective uncertainties can be presented in the following table:

Lemon Juice Sample

Average Volume of Lemon Juice Sample Titrated (cm3)

Fresh Lemon Juice

32.60 ± 0.20

10 minutes boiled lemon juice

41.25 ± 0.65

1 hour boiled lemon juice

30.05 ± 2.85

Table 3 the average volume of lemon juice sample titrated to decolourise DCPIP together with their respective uncertainties.

According to international data standard (July 1980), the mass of ascorbic acid equivalent to 1 cm3 of DCPIP is 0.05 mg. This means that the volume of lemon juice sample titrated to decolourise DCPIP contains 0.05 mg of ascorbic acid.

Let Cs be the concentration of ascorbic acid in diluted lemon juice sample.

Therefore, Cs = 0.05 mg / volume of diluted lemon juice sample titrated.

Let Cj be the concentration of ascorbic acid in pure lemon juice sample.

Hence, a formula can be derived as follows:

CjCs = concentration of ascorbic acid in pure lemon juice sampleconcentration of ascorbic acid in diluted lemon juice sample

It is assumed that the concentration of ascorbic acid in pure lemon juice sample is 100%. Therefore, the concentration of ascorbic acid in diluted lemon juice sample is 4% (because 4 cm3 of pure lemon juice sample is diluted in 100 cm3 of solution). Hence:

CjCs = 100%4%

Cj = 25 × Cs

Where Cs = 0.05 mg / volume of diluted lemon juice sample titrated.

From the formula derived, the concentration of vitamin C in lemon juice sample (Cj) can now be calculated. Below is an example of calculation made to find the concentration of vitamin C in fresh lemon juice.

Cj (fresh lemon juice) = 25 × 0.05 mg32.60 cm³

= 3.834 × 10-2 mg cm-3

Uncertainties:

Percentage uncertainty of volume of diluted lemon juice titrated = 0.10 cm³32.60 cm³ ×100%

= 0.307%

Percentage uncertainty of DCPIP = 0.1 cm³1.0 cm³ ×100%

= 10.0 %

Total percentage uncertainty of the concentration of vitamin C in fresh lemon juice,

= 0.307% + 10.0 %

= 10.307%

Therefore, the concentration of vitamin C = 3.834 × 10-2 mg cm-3 ± 10.31%

in fresh lemon juice

Hence, the concentration of vitamin C in all samples of lemon juices can be presented in the following table:

Lemon Juice Sample

Concentration of Vitamin C (×10-2 mg cm-3)

Fresh lemon juice

3.834 ± 10.31%

10 minutes boiled lemon juice

3.030 ± 10.24%

1 hour boiled lemon juice

4.160 ± 10.33%

Table 4 the concentration of vitamin C in different lemon juice samples.

The data in table 4 can be analysed much clearer by plotting a bar chart.

Graph 1 the concentration of vitamin C in lemon juice samples

DISCUSSION

2,6 – dichlorophenolindophenol (DCPIP) is a blue chemical compound widely used as redox dye. The blue colour indicates that it is in oxidised condition, while it will change to colourless when reduced. DCPIP is widely used as Vitamin C indicator. The presence of vitamin C (a good reducing agent) will cause DCPIP to change its colour from blue to pink due to acidic condition, and eventually turn colourless compound after being reduced by ascorbic acid. Below are the chemical equations involved in the reaction:

DCPIP (blue) + H+ ———-> DCPIPH (pink)

DCPIPH (pink) + VitC ———-> DCPIPH2 (colourless)

C6H8O6 + C12H7NCl2O2 ———-> C6H6O6 + C12H9NCl2O2

Vitamin C is one of the least stable of all vitamins in solution and is oxidised readily in light, air and when heated. It is also water soluble. This means that heating in water, (like cooking broccoli in boiling water) causes the vitamin to leach out of the food into the water and also to be oxidised, first to dehydroascorbic acid and then to diketogulonic acid. This last compound has no Vitamin C activity at all and is irreversible. So, the longer the food is cooked and the higher the temperature is used, the more oxidation of vitamin C occurs thus reducing the concentration of vitamin C. As a matter of fact, cooking can reduce the Vitamin C content of vegetables by around 60% possibly partly due to increased enzymatic destruction as it may be more significant at sub-boiling temperatures. In this experiment, a sample of lemon juice is boiled for 10 minutes while another sample for an hour. A longer cooking time will cause more vitamin C molecular structures to be destroyed, which result to a low vitamin C concentration.

From the bar chart plotted, it can be observed that 1 hour boiled lemon juice contains the highest concentration of vitamin C, followed by fresh lemon juice and 10 minutes boiled lemon juice. However, this contradicts with the theory, in which fresh lemon juice is supposed to have the highest concentration of vitamin C, while 10 minutes boiled lemon juice should be the second followed by an hour boiled lemon juice. This is because during the experiment, the 1 hour boiled lemon juice sample is used straight away right after being boiled instead of leaving it to cool down for a while. Since DCPIP is highly sensitive to heat, the warm diluted lemon juice sample titrated will cause the molecular structure of DCPIP to alter, thus causing it to decolourise faster (in fact, much faster than the fresh lemon juice). This explains why the trend observed is not the same with the theoretical trend.

CONCLUSION AND EVALUATION

1. Conclusion

1 hour boiled contains the highest vitamin C concentration, followed by fresh lemon juice and 10 minutes boiled lemon juice. Hence, it can be concluded that the longer the cooking time of lemon juice, the lower the concentration of vitamin C. However, when the lemon juice is cooked for a long period of time, i.e. 1 hour, the concentration of vitamin C will be high. This situation contradicts with the theory explained in the discussion part, and may be due to some limitations of the experiment that will be discussed later on.

Therefore, the hypothesis is rejected.

1. Evaluation

Limitations

Suggestions

DCPIP is a very sensitive chemical compound that can easily be reduced. The slow process of titration carried out could cause the DCPIP to reduce due to some other factors instead of the presence of ascorbic acid.

The conical flask which contained DCPIP should not be shaken too vigorously during the titration since it aids the reduction reaction of DCPIP. Apart from that, make sure that the conical flask is free of water and the DCPIP should be transferred into the conical flask by flowing it to the wall of the flask.

Due to time constraint, the boiled lemon juice sample is used straight away right after being boiled instead of leaving it to cool down for a while. DCPIP is very sensitive to heat, thus the warm sample titrated into DCPIP will cause the DCPIP to decolourise faster.

The sample should be left about 10 minutes for it to cool down before being used in the titration. This will ensure that the DCPIP is in optimal condition thus no other factor will cause it to decolourise faster other than because of the presence of ascorbic acid.

The dilution of lemon juice samples is done in a 100 cm3 measuring cylinder. It is quite difficult to shake the solution inside a measuring cylinder (a step which is supposed to be done in dilution process) and the huge uncertainty of measuring cylinder could greatly affect the reliability of the data obtained.

The dilution process should be done in a volumetric flask since it has a smaller uncertainty and the apparatus itself is invented and meant for dilution.

The three lemon juice samples are prepared from different lemon fruits. Problems and inaccuracies might arise as different lemon fruits contain different concentration of vitamin C thus causes the data obtained to be defective.

The lemon fruits should be prepared together. Then, all the lemon juice prepared should be mixed together before being separated into 3 samples. This will ensure that all the three samples have relatively the same.

REFERENCES

1. Nutrition in Perspective, Patricia A. Kreutler

2. Nutrition Concepts and Controversies, Sizer and Whitney

1. http://www.en.wikipedia.org

2. Campbell, Reece. Biology Eighth Edition. Pearson Benjamin Cummings, CA (2008). Page 877.